The invention relates to a system for the transmission of information between a plurality of stations which are interconnected via a bus, consisting of at least one conductor, and transmit, via said bus, information encoded in the form of different signal levels.
A system of this kind is already known, for example from DE 195 23 031 A1.
Therein, a station is essentially in one of two different modes. In the normal or active mode the communication is possible between all stations, i.e. all stations can correctly receive a message transmitted by any other station, a transmitting station imposing the appropriate signal level on the bus by means of a bus driver. In the sleep mode a sleep level occurs on the bus and communication is not possible; however, a message transmitted by an arbitrary station can be interpreted by the other stations as a request for changing over the system to the normal mode, so that via the bus the system can be woken up from the sleep mode and communication can be resumed.
Typical applications of such systems involve a large number of stations so that there is a need to save electric power, for example in vehicles in the parked state, so that the sleep mode should be entered as often and for as long as possible. However, at the same time there is usually a need that some of the stations would occasionally like to communicate with one another also in the sleep mode. The temporary normal operation of the overall system with all stations would not be acceptable for this purpose, considering the power consumption.
It is an object of the invention to enable sub-system operation in which some of the stations communicate with one another, whereas the remaining part of the stations remains in the sleep mode without interpreting the ongoing communication by some of the stations as a wake-up request.
This object is achieved according to the invention in that in addition to the signal levels there is defined an additional wake-up level which is situated outside the signal level range and can hence be clearly distinguished from the signal levels for all stations. If this wake-up level does not occur for a communication, the stations already in the sleep mode will remain in the sleep mode.
In order to achieve a sub-system operation starting from a sleep mode, first all stations are briefly woken up by the transmission of a wake-up level. This wake-up level normally succeeds a data message. If desired, the message can define the subscribers for the sub-system operation, so that the stations which are not addressed can immediately enter the sleep mode again. The further communication in the sub-system operation takes place on the basis of signal levels.
The invention also relates to a station which includes at least one receiver for receiving signals received via a bus, the station being switchable to the normal or active mode by way of a wake-up signal having a voltage level which lies outside the signal level range.
In order to avoid mix-ups and enhance the system consistency, in the stations the wake-up level can be interpreted logically identically to a signal level, so that during normal operation a message transmitted by a station by means of a bus driver which unduly operates with wake-up levels is correctly received by all stations in the same way as if the bus driver had used the normal signal levels.